UC Berkeley

Iron plays a central and critical role in the water oxidation mechanism and the activity of transition-metal oxides and (oxy)­hydroxides. Tracking Fe dynamics (deposition/dissolution/electrolyte transport) and unraveling the chemistries of various Fe active sites under oxygen-evolution reaction (OER) conditions are important for catalyst design, particularly for applications in alkaline electrolysis. Here, we use CoO x H y thin films as a platform to investigate Fe transport and reactivity at the catalyst-electrolyte interface and its impact on OER activity. We find that the deposition/dissolution of the surface-absorbed Fe species is governed by the transport of soluble Fe species and applied potential. Soluble Fe species in the electrolyte adsorb on CoO x H y under stirred electrolyte conditions. Accelerated Fe desorption is observed with a more-positive OER potential. The surface-localized Fe sites generated by absorption from soluble Fe species have a higher OER turnover frequency (TOFFe) compared to Fe in codeposited CoFeO x H y films. Operando X-ray absorption spectroscopy shows structural similarity between reference Fe oxyhydroxides and surface Fe sites on CoO x H y , contrasting with Fe sites within the CoO x H y structure made by codeposition, where Fe shows a different apparent X-ray absorption edge energy. The OER activity of the surface-absorbed Fe decreased by Fe desorption but was recoverable by redepositing Fe species under non-OER conditions.